Method of forming shell molds and composition therefor



METHOD OF FORMING SHELL MOLDS AND COMlGSITION THEREFOR Stanford .l. Hetzel, Cheltenham, Pa., 'assignor to Sun Oil Company, Philadelphia, 2921., a corporation of New Jersey No Drawing. Application July 16, 1954, Serial No. 443,967

Claims. (Cl. 22-193) This invention relates to the preparation of sand molds which are useful in foundry metal casting procedures.

According to the present invention, limed naphthenic acid is used as a sand bonding agent in the formulation of foundry sand molds.

Naphthenic acids are recovered from mineral oil by procedures which are well known to those skilled in the art of petroleum refining. For use in this invention, the naphthenic acids are limed, i. e. fused with calcium hydroxide at elevated temperature, to form a reaction product which has strong binding characteristics. The limed naphthenic acid product is thoroughly admixed with foundry sand and heated to a sufficiently high temperature to liquefy the limed product. Upon liquification, the limed naphthenic acid coats the particles of sand ,admixed'therewith to form a plastic compositionwhich may be shaped and hardened into a mold useful in metal casting operations.

In conventional foundry casting procedures, molten metal is formed into desired shapes by pouring the metal into a sand-clay mold and allowing the metal to cool and solidify in the shape of the mold. The mold is formed by tamping damp said and clay around a wooden or metal pattern, and then carefully removing the pattern from the packed sand-clay composite having a sharp pattern impression. Generally these molds, being completely supported in a box or pit, are strong enough without the addition of a supplementary binder to withstand contact with molten metal and the subsequent cooling thereof.

In all but the simplest castings, however, it is necessary to suspend core molds in the molds described above in order to provide additional cavities and hollow spaces which are required in the cast metal product. These core molds comprise mainly foundry sand with the addition thereto of a minor proportion of a suitable material which strongly binds the sand in the desired core shape.

i In one embodiment of the present invention, foundry core molds are prepared by utilizing limed naphthenic acid as a sand binder.

, In the selection of a suitable core bonding material, there are a number of requirements which must be considered. The binder must be liquefiable and capable of evenly coating foundry sand particles to form a soft mixture which can be shaped into. the desired mold form. The binder must be hardenable, and when hardened should impart sufficient strength to the core to enable the core to withstand deformation during storage and setting up in the mold, and after contact with molten metal. Upon contact with the cast metal, the binder shouldgradually decompose so that whensolidification of the cast metal is completed, the core has become suflicientlycrumbly to be readily removable as by shaking out from the cast metal. Still another factor to be considered is the porosity of the sand core. The binder must bond sand particles to form a core which is sufficiently porous to permit gases evolved as by decomposition of the binder during the contact with the heated metal, to

Patented Nov. 12, 1957 escape by passage through the core. In this way, the formation of pressure pockets in the core and the possibility of blowing holes in the casting can be avoided.

I have found that limed naphthenic acid fulfills the above enumerated requirements of a core binder. In this embodiment of the invention, foundry sand is thoroughly admixed with the limed acid to form a uniform mixture comprising a minor amount of the limed naphthenic acid, e. g. less than about 10% by weight and preferably about 1 to 5%. This core mixture is heated to a temperature sufficiently high to liquefy the limed naphthenic acid binder, thereby to obtain a plastic mixture of sand coated with the binder. The plastic mixture is formed into the desired core shape as by molding in a suitable pattern mold, and is hardened by baking or, depending on the softening point of the binder, simply by cooling to form a rigid core which is suitable for use in foundry casting operations. During the shaping operation, the use of excessive pressures should beavoided in order to insure that the resulting mold is sufiiciently porous for use in metal casting procedures. The use of minor amounts of the limed naphthenic acid in formulating the core is important from the standpoints both of economy and core porosity which decreases as the amount of binder is increased. The core mold formed as described above, doesnot absorb substantial quantities of water and may be storedfor a considerable length of time without loss of strength or danger of deformation.

When used in a casting operation, the core is suspended ina mold by means Well known in the art, and molten metal is poured into the mold and around the core. The limed naphthenic acid core binder gradually decomposes upon exposure to the heated metal, and decomposition gases escape by passage through the porous core. When solidification and cooling of the cast metal is completed, the core is generally sufiiciently depleted in strength due to decomposition of the binder as to be readily removable from the cast metal by rapping and shaking out.

In a second embodiment of my invention, foundry shell molds are prepared by utilizing limed naphthenic acid as a sand binder.

Shellmolding is a fairly recent foundry development which provides high precision metal castings by a process which is readily adaptable to automatic production methods. Fine, dry sand is admixed with a suitable pulverized bindery This mixture is placed in a dump box and a heated metal plate comprising a pattern of part of the desired mold is fastened to the top of the box. The box is inverted allowing the sand-binder mixture to come into Contact with the heated pattern. Th binder nearest the pattern melts, thereby coating the sand particles and forming a plastic sand binder shell in the contour of the pattern plate.

The dump box is returned to its original position, permitting excess sand-binder mixture to drop to the bottom of the'box andleaving a soft shell adhering to the pattern. The thickness of this she'll will depend upon the temperature of the pattern plate and the duration of the contact betweenv the pattern and sand-binder mixture. The pattern withv its adhering shell is removed from the dump box and cured at an elevated temperature and pressure to harden the shell, The hardened shell is stripped from the pattern and then may be fastened to a corresponding shell to form acomplete shell mold. The mold is suitably supported as by being imbedded in metal shot and molten metal cast therein. a

, Asin the case of the 'core binder, a number of requirements must be considered in the selection of a suitable shell bonding agent. The binder should be normally solid and capable of being stored in admixture with sand Without evidencing a tendency to prematurely coat the sand and form lumps which would cause uneven binder distribution in the actual mold formation. Upon contact with the heated pattern plate, the binder must readily liquefy and evenly coat and bind together the sand particles. The binder must harden to bind the shell into a rigid shape, and the thusly formed shell must then be sufficiently strong to retain its dimensions after storage, setting up, and contact with molten metal. Finally, the shell must be sufficiently porous to permit the escape therethrough of vapors formed during the casting as by decomposition of the binder.

I have found that limed naphthenic acid is particularly suitable as a sand binding agent in shell molding processes. The limed naphthenic acid may comprise the sole shell binder, although in preferred practice a thermosetting plastic resin is used in conjunction with the limed product to form the shell binder. Use of the thermosetting resin facilitates curing and hardening the shell mold. In this preferred practice, the limed naphthenic acid acts not only as a binder but also as a sand wetting agent for the thermosetting resin and thus insures a more even distribution of the binder and a more uniform shell strength.

The limed naphthenic acid comprises only a minor proportion ofthe shell mold, for example about 1-10% by weight. Amounts of binder in excess of about 10% are generally undesirable and tend to cause the shell to have too low a porosity. When used as the sole binder, limed naphthenic acid preferably comprises about to by weight of the shell mold. When both limed naphthenic acid and thermo setting resin are used as binders, a preferred shell composition comprises about 2 to 5% limed naphthenic acid, 2 to 5% thermosetting resin, and the remainder clean foundry sand.

In an exemplary practice of preparing a shell mold according to the invention, limed naphthenic acid is thoroughly admixed with phenol-formaldehyde resin and clean, dry foundry sand .of 80 to 200 A. F; S. fineness. The proportions by weight of the resulting mixture are about 2 to 5 parts of limed naphthenate, 2 to 5 parts of phenol-formaldehyde resin, and 90 to 96 parts of sand.

This mixture is placed in a dump box, and a metal pattern plate which previously has been heated to a temperature of about 400 F. to 550 F. is atfixed to the top of the box. The dump box is inverted for a short time to allow a portion of the sand-binder mixture to contact the pattern. The binder melts during this contact, coating adjacent sand particles, and there is formed a soft shell which adheres to the pattern plate and seeks the contours of the pattern.

The dump box is returned to its original position to separate excess sand and binder, and the pattern plate with its adhering soft shell is removed from the box. The shell is cured while in contact with the pattern at a temperature in the range of about 300 F. to 600 F. and at an advanced pressure as for example 90 p. s. i. g. thereby to obtain a hardened shell mold. The shell mold is then stripped from the pattern and may be used directly or stored for future use. The thusly formed shell is strong enough to withstand normal treatment during storage, setting up, and casting, and is sufficiently porous to permit the passage therethrough of gases formed during contact of the mold with heated metal.

When used in foundry casting, the shell mold is gen erally placed in a box and backed with metal shot; molten metal is then poured into the mold. Upon solidification and cooling of the metal, a casting of high precision is obtained.

The novel foundry sand binder of this invention is formed by fusing naphthenic acid with calcium hydroxide at a temperature sufficiently high to promote the reaction between these constituents. Generally temperatures in the range of about 300 F. to 400 F. are suitable. Water, which is a product of the fusion reaction, is driven otf at the reaction conditions, leaving a molten reaction product which is predominately calcium naphthenate and which solidifies upon cooling to form the limed product which is the foundry sand binder of this invention.

The naphthenic acids which are limed as above indicated may be any commercially available naphthenic acid fraction with an acid No. in the range of about to 250 mg. of KOH per gram. Prior to fusion with calcium hydroxide, these naphthenic acids are de-oiled. Complete de-oiling is not essential but the oil content of the naphthenic acids should be reduced preferable to below about 10%.

The characteristics of the limed product are dependent upon the properties of the naphthenic acids treated, and upon the relative amounts of calcium hydroxide and naphthenic acid used in the reaction. Higher ratios of calcium hydroxide to naphthenic acid result in a limed product having stronger binding characteristics and a higher melting point. It is generally desirable to fuse naphthenic acids with an amount of calcium hydroxide equivalent to that needed to react with a predominance and preferably with all of the naphthenic acids, although less calcium hydroxide may be utilized on some occasions.

Use of higher acid No. naphthenic acids is preferred since these acids may be fused to form a product which has a higher calcium content and superior binding qualities. Normally, naphthenic acids with an acid No. above about mg. of KOH per gram are preferred although as above indicated naphthenic acids with a lower acid No. may successfully be utilized.

A reactant ratio in the range of about 2:1 to 10:1 parts by weight of naphthenic acid to calcium hydroxide will. normally be adequate to produce a limed product which is suitable as the foundry sand binder in the practice of this invention.

I claim:

1. In the formulation of sand molds which are useful in foundry metal casting procedures, the steps which comprise: forming a uniform mixture comprising a major proportion of foundry sand and a minor proportion of limed naphthenic acid, heating this mixture to a sufficiently high temperature to liquefy the limed naphthenic acid, shaping this resulting mixture into a desired sand mold shape, and hardening the shaped mixture to form a sand mold.

2. In the formulation of foundry shell molds, the steps which comprise: forming a uniform mixture comprising a major proportion of foundry sand and minor proportions of limed naphthenic acid and thermosetting resin, contacting said mixture with a heated pattern plate thereby to liquefy limed naphthenic acid and thermosetting resin and to form a soft, adhering shell of said mixture in the contour of the pattern plate, and curing this shell at advanced temperature and pressure thereby to form a hardened shell mold.

3. Foundry sand mold composition comprising a major proportion of foundry sand and a minor amount of limed naphthenic acid.

4. Foundry shell mold composition comprising a major proportion of foundry sand and 5% to 10% by weight of limed naphthenic acid.

5. Foundry shell mold composition comprising a major proportion of foundry sand, 2% to 5% by weight of limed naphthenic acid, and 2% to 5% by weight of thermosetting resin.

References Cited in the file of this patent UNITED STATES PATENTS 2,091,020 Shipp Aug. 24, 1937 2,328,622 Crawford Sept. 7, 1943 FOREIGN PATENTS 674,421 Great Britain June 25, 1952 700,763 Great Britain Dec. 9, 1953 

1. IN THE FORMULATION OF SAND MOLDS WHICH ARE USEFUL IN FOUNDRY METAL CASTING PROCEDURES, THE STEPS WHICH COMPRISE: FORMING A UNIFORM MIXTURE COMPRISING A MAJOR PROPORTION OF FOUNDRY SAND AND A MINOR PROPORTION OF LIMED NAPHTHENIC ACID, HEATING THIS MIXTURE TO A SUFFICIENTLY HIGH TEMPERATURE TO LIQUEFY THE LIMED NAPHTHENIC ACID, SHAPING THIS RESULTING MIXTURE INTO A DESIRED SAND MOLD SHAPE, AND HARDENING THE SHAPED MIXTURE TO FORM A SAND MOLD. 